Wideband Code Division Multiple Access (WCDMA) has been chosen as the radio technology for the paired bands of the UMTS. Consequently, WCDMA is the common radio technology standard for third-generation wide-area mobile communications. WCDMA has been designed for high-speed data services and, more particularly, Internet-based packet-data offering up to 2 Mbps in indoor environments and over 384 kbps for wide-area.
The WCDMA concept is based on a new channel structure for all layers built on technologies such as packet-data channels and service multiplexing. The new concept also includes pilot symbols and a time-slotted structure which has led to the provision of adaptive antenna arrays which direct antenna beams at users to provide maximum range and minimum interference. This is also crucial when implementing wideband technology where limited radio spectrum is available.
The uplink capacity of the proposed WCDMA systems can be enhanced by various techniques including multi-antenna reception and multi-user detection or interference cancellation. Techniques that increase the downlink capacity have not been developed with the same intensity. However, the capacity demand imposed by the projected data services (e.g. Internet) burdens more heavily the downlink channel. Hence, it is important to find techniques that improve the capacity of the downlink channel.
Bearing in mind the strict complexity requirements of terminals, and the characteristics of the downlink channel, the provision of multiple receive antennas is not a desired solution to the downlink capacity problem. Therefore, alternative solutions have been proposed suggesting that multiple antennas or transmit diversity at the base station will increase downlink capacity with only minor increase of complexity in terminal implementation.
The transmit diversity concept adopted for the FDD (Frequency Division Duplex) mode of third generation WCDMA system in the 3G standardization is currently being optimized for the case of two transmitting antennas at the base station.
In case of a so-called open-loop mode, a space-time block code is applied for the two transmitting antennas. The channel symbols are divided into two-element blocks which are transmitted from a first and second antenna, respectively, at successive time instants. These symbols are transmitted using the same spreading code. The receiver then uses a linear orthogonal processing based on the estimated channel coefficients to detect the transmitted symbols.
Alternatively, in case of a so-called closed-loop mode, a weight information is fed back from the terminals to the base station to approximate matched beamforming. FIG. 1 shows an example of such a closed-loop or feedback mode for a downlink transmission between a base station (BS) 10 and a mobile terminal or mobile station (MS) 20. In particular, the BS 10 comprises two antennas A1 and A2, and the MS 20 is arranged to estimate the channel on the basis of pilot channel signals used to probe the downlink channel and transmitted from the two antennas A1 and A2. Then, the MS 20 feeds back the discretized channel estimate to the BS 10. The antennas (or antenna elements) A1 and A2 are spaced sufficiently close to each other, so that the propagation delays between each of the antennas A1 and A2 and the MS 20 are approximately identical (within a fraction of a duration of a chip of the WCDMA spreading code). This is important in order to maintain downlink orthogonality in a single-path channel. Naturally, it is desired to develop a robust and low-delay feedback signaling concept.
Transmit diversity techniques provide a low-cost solution to increase downlink capacity in third generation systems. A number of different transmit diversity concepts have been developed. Both open and closed-loop concepts have significant merits in different environments and with different service assumptions.
In WCDMA, different modes have been suggested for the closed-loop concept which is optimized for two antennas. In the Selective Transmit Diversity (STD) mode, one bit per time slot is used to signal the “best” antenna from each terminal. The MS 20 estimates channel coefficients from common pilot signals (antenna or beam specific), selects the stronger antenna (two possibilities), and sends the index to the BS 10 using a 1.5 kbps subchannel. Thus, a simple dedicated channel estimate can be derived from continuous common channel estimates. In the STD mode, the bit length of the feedback signaling word is one bit. The feedback bit rate is 1500 bps and the feedback signaling word is used for controlling the power supplied to the antennas A1 and A2.
Furthermore, modes 1 and 2 (referred to as Transmission Antenna Array (TxAA) modes) are suggested with a slower feedback link, where feedback weights used for controlling power and/or phase of the transmission signals of the antennas A1 and A2 are modified after a certain number of slots. In particular, a quantized feedback is signaled to the BS 10 using the 1.5 kbps subchannel. In mode 1, the possible Tx feedback weights are selected from a QPSK constellation. In mode 2, the possible Tx feedback weights are selected from a 16-state constellation.
FIG. 2 shows a table indicating characteristic parameters of the above modes. In particular, NFB designates the number of feedback bits per time slot, NW the number of bits per feedback signaling word, Na the number of feedback bits for controlling an amplification or power at the antennas A1 and A2, and Np the number of feedback bits for controlling a phase difference between the antennas A1 and A2. As can be gathered from the table of FIG. 2, one bit is fed back per time slot in each of the feedback modes.
In the Tx AA mode 1, the feedback signaling word comprises two bits, and an update is performed after both feedback bits have been received, i.e. after two time slots. The feedback signaling word is only used for controlling the phase difference between the two antennas A1 and A2.
In the Tx AA mode 2, the bit length of the feedback signaling word is four, and an update is performed every four time slots. In particular, one bit of the feedback signaling word is used for controlling the amplification (power) at the antennas A1 and A2, and three bits are used for controlling their phase difference.
The required channel estimates are obtained e.g. from the common pilot channel signal transmitted with a known power from each antenna. In WCDMA systems, rather accurate estimates can be obtained by using the common channel pilots (CPICH) transmitted continuously from the two antennas A1 and A2. The feedback information can be transmitted in the Feedback Signaling Message (FSM) as a part of the FBI field of the uplink dedicated physical control channel (DPCCH)
It is to be noted that the STD mode may be implemented in an analogous manner in the beam domain. In this case, the MS 20 signals to the BS 10 whether to rotate channel symbols transmitted from the antenna A2 by 180°. In this case, the BS 10 transmits simultaneously from both antennas A1 and A2. Thus, the phase difference between the antennas A1 and A2 is switched between 0° and 180° in response to the feedback value.
In the TxAA modes 1 and 2, the MS 20 transmits estimated and quantized channel parameters to the BS 10 which then weights the transmitted signals accordingly. Thus, a higher resolution than 180° (as provided by the STD mode) can be achieved. The MS 20 selects the Tx weight (or Tx beam) from 4 or 16 different constellations, respectively.
As regards the table of FIG. 2, it is to be noted that an equal power is applied to the antennas A1 and A2 in each case where Na=0. Furthermore, the antennas A1 and A2 are uniquely defined by their respective pilot codes of the CCPCH (Common Control Physical Channel) of the UMTS. The derived amplitude and phase applied to the antennas A1 and A2 is called a weight and the set of weights is grouped into a weight vector. Specifically, the weight vector for the present case of two antennas is given by
            w      _        =          [                                                  PA1                                                                                          PA2                            ·                              exp                ⁡                                  (                                      j                    ⁢                                                                                  ⁢                    π                    ⁢                                                                                  ⁢                    Δ                    ⁢                                                                                  ⁢                                          ϕ                      /                      180                                                        )                                                                        ]        ⁢        wherein Δφ denotes the phase difference (phase weight) fed back to the BS 10. If more than two antennas, i.e. an antenna array, is used, the dimension of w becomes larger than two. In this case, a directional antenna may be achieved by using relative phases between antennas. The estimated phase of the feedback signal in the complex plane is then used for controlling the transmit direction.
Both TxAA modes 1 and 2 rely on the use of dedicated pilots. As it is possible that some future frame formats do not include dedicated pilots, the concept of Soft-Weighted Space-Time Transmit Diversity (SW-STTD) was suggested. In SW-STTD, relative weighting factors are imposed on the signals transmitted from the antennas A1 and A2. The stronger antenna (as measured from the CPICH) is set to have the larger weight. The CPICH power ranking can be signaled with 1 bit/slot feedback as specified in A. Hottinen et al, “Transmit diversity by antenna selection in CDMA downlink”, Proc. IEEE International Symposium of Spread Spectrum Techniques and Applications (ISSSTA), Sun City, South Africa, September 1998. For optimal reception, the receiver needs to know precisely the weights that are applied at the transmitter. To achieve this, STTD processing is used in decoding the SW-STTD transmission at the terminal, thus applying effectively equal gain combining to the two transmit antennas. Furthermore, the transmit weight may be changed based on two successive feedback commands rather than for each command independently. For example, when the base station estimates feedback bits to be [1 1] or [0 0], the transmit gains are set to [g1 g2], where g1>g2 or g1<g2, respectively. When the successive feedback bits are different, i.e. [1 0] or [0 1], equal power (STTD) transmission is applied, where g1=g2. For simplicity, the actual values of the asymmetric weights may be fixed e.g. to √{square root over (0.8)} and √{square root over (0.2)}, which are applied also in the above TxAA mode 2. The channel estimate is obtained by filtering the CPICH of the two antennas, as in the open-loop mode.
In the IMT-2000 specifications, a two-dimensional space-time code and a closed-loop feedback-based array control are applied as described e.g. by S. M. Alamouti, “A simple transmit diversity technique for wireless communications”, IEEE Journal on Selected Areas in Communications, Vol. 16, No. 8, pp. 1451–1458, October 1998. The current concept is defined for a two-element array (at the base station) which does not require any array calibration. The feedback control is signaled from each terminal and individual channel-matched “beams” are transmitted to minimize the transmit power while meeting the signal-to-interference requirements. The gains achievable by the feedback mode are significant (when compared to open-loop or single-antenna transmission) especially in low mobility environments.
Extensions to the above two-element concepts are considered in order to further increase the system capacity in low mobility environments. While many of these extensions are straightforward, most of them are applicable only in very low mobility environments, since the accuracy (or delay) of the feedback signaling is often compromised.